1. Field of the Invention
The invention relates to a method of deriving an assembled image from successive X-ray images. The invention also relates to an X-ray examination apparatus which is suitable for deriving an assembled image from successive X-ray images.
2. Description of Related Art
A method and an X-ray examination apparatus of this kind are known from European patent application EP 0 655 861.
According to the known method the successive X-ray images are picked up by means of an image intensifier pick-up chain which includes an X-ray image intensifier and a television camera. A series of successive X-ray images of the relevant parts of the patient is formed on an entrance screen of the X-ray image intensifier by irradiating separate parts of an object, for example a patient to be radiologically examined, by means of an X-ray beam; these X-ray images are converted into successive optical images which appear on the exit window of the X-ray image intensifier and electronic video signals are derived from the successive optical images by means of the television camera. An image processing unit forms an image signal representing the assembled image from the electronic video signals. The assembled image reproduces the individual parts of the patient together in a single image.
Because the known method utilizes an X-ray image intensifier, merely a comparatively narrow strip at the center of the successive individual X-ray images can be used. The use of the X-ray image intensifier introduces distortions in the optical image; these distortions are pronounced notably at the edge of the optical image. One cause of the distortions is the cushion distortion in the X-ray image which is due to the curvature of the entrance screen. Despite the fact that only narrow strips at the center of respective X-ray images, where the distortion is lowest, are used, according to the known method extensive operations must be performed so as to produce an assembled image which does not contain an excessive amount of distortion. Because only narrow strips of the successive X-ray images can be used, the known method requires a comparatively large number of X-ray images so as to form an assembled image of an elongate part of the patient. Notably peripheral angiography, being the imaging of blood vessels in an arm or a leg of a patient, requires the formation of a large number of X-ray images which may even reach a few dozen. Consequently, the patient to be examined is exposed to a rather high X-ray dose.